The present invention relates to an apparatus and method for establishing the proper gas flow to efficiently and effectively unload materials from a storage location using a pressurized or vacuum conveying system, respectively. The present invention is ideally suited to set a proper gas flow in conveying systems used to unload friable, abrasive, or degradable materials from tank trucks.
Plastic pellets are commonly transported from the facility at which the pellets are manufactured to bulk plastic consumers using a tank truck. Referring to FIG. 1, a common transport system will include a tank truck T, or bulk truck, which has conical hoppers H used to store the plastic pellets (not shown). To unload the truck T, a gas stream, usually air, is directed through a pipe, called a convey line C, below the conical hoppers H. The pellets are introduced into the gas stream by pressurizing the tank trailer TT and opening a hopper valve HV that separates each hopper H from the convey line C.
Still referring to FIG. 1, a positive displacement blower B, which is usually located on the tractor portion of the truck, drives the airflow in the convey line C. The blower B generates an air velocity proportional to the speed of the truck engine (not shown). The air velocity at the product pick-up point should only be great enough to entrain the solids dependably so that minimum damage is done to the solids and the unloading rate is maximized for the allowable convey line pressure.
The unloading process, however, is complicated by the fact that bulk trucks vary in performance and design, even among trucks made by the same manufacturer. For example, and as known, the tractor portion of the truck is assembled from many components manufactured by different companies, such as the transmission, the blower, and the filters. The bulk trailers, likewise, may have different pipe sizes, piping arrangements, valves, filters, and, optionally, coolers. Thus, bulk trucks lack a standard arrangement in the industry and may thus have any of a number of blower models and gearing ratios between the engine of the truck and the blower.
Further complicating the process, each different type of plastic pellet transported by the trucks has a unique entrainment velocity as well as an optimum conveying velocity for both acceptable convey rate and product degradation. The speed of the positive displacement blower, which is established by the speed of the truck engine through gearing with a predetermined power take-off ratio, determines the amount of air moved and, therefore, the gas velocity for a given pipe size.
In addition to blower speed, the velocity of the airflow is also a function of the air pressure within the pipe. System pressure results from the resistance of the system to the flow of the gas and the entrained product. That is, resistance to the flow of air and the entrained product creates a pressure differential between the two ends of the convey line. As such, the more plastic pellets that flow through the pipe at one time, the higher the resistance and thus the higher the pressure required to maintain the airflow therethrough. However, the pressure in the system compresses the air, and the resulting reduction in gas volume tends to reduce gas velocity. Accordingly, at a constant airflow, a higher pressure results in compression of the air, causing the air velocity to be lower.
The actual convey rate of the plastic pellets is determined by the allocation of the pressure resource shared between the density of the pellets flowing in the pipe and the velocity of the pellets. Thus, the operating pressure must be known or assumed before it can be determined what flow of gas will produce the desired velocity. The prior art systems do not adequately address these engineering considerations to unload bulk solids from tank trucks as efficiently as possible with the least possible damage to the material.
In the prior art systems of unloading bulk trucks, the driver first selects an operating pressure based on an acceptable temperature and the system pressure. The driver then selects a blower speed by setting the engine speed to move the amount of air that will produce the optimum velocity for the product to be handled at the desired operating pressure. To assist the operator, tables are available to select the desired pressures and velocities for the system. For example, Table 1 lists some appropriate conveying velocities and pressures:
After the operator selects the proper velocity and unloading pressure from the appropriate table for the product to be moved, he or she then determines the proper engine speed based on the blower frame and the power take-off from another table. For example, Tables 2 through 5 below illustrate engine settings for popular blowers driven by three different power take-off ratios to produce three different velocities at specific operating pressures for a few plastic materials:
After considering the applicable charts and starting the blower at the tabulated speed, the operator then adjusts the trailer valves to develop the desired operating pressure.
As one skilled in the art will appreciate, the listed tables are illustrative, and many more tables would be required to include all possible combinations and factors relevant in determining the proper unloading airflow velocity. For example, additional charts would need to address filter pressure drop and unloading pressure for each truck, in addition to more tables showing other combinations of unloading line power take-off ratios for different blower sizes and manufacturers.
In actual practice, however, the operators do not always wade through the numerous charts to tabulate the appropriate speed. Instead, engine speed is often set at the discretion of the truck driver at a value that he thinks will unload the truck at the highest rate. The driver seldom knows what blower model or power take-off ratio is installed on the truck, and he often lacks any specific training or any written procedures for loading and unloading the products. Further exacerbating the situation, it may be counterintuitive to some drivers that using a lower blower speed can actually result in unloading a bulk truck quicker than a higher blower speed. The unfortunate result of using a less efficient higher blower speed to convey plastic pellets is the generation of more fines and streamers when unloading the pellets. Another consideration is that compression of the air also increases the air temperature, which may result in product damage unless the maximum pressure is limited or a gas cooler is added.
The result of the prior art practice frequently is excessive product damage, the most common complaint from bulk plastics customers, and less than optimum unloading rates. Accordingly, there is a need in the art to allow operators to easily and accurately determine the correct conveying velocity for unloading bulk materials from a tank truck.
The present invention overcomes the drawbacks in the art and provides a device and a method that allows bulk truck engine speed (in RPM), which drives the blower, to be quickly and accurately set for the optimum gas flow for unloading any product for which the minimum entrainment velocity is known. The device is installed on the unloading pipe of the trailer, and a truck engine RPM is determined that produces a tabulated system pressure specific to the solid being handled. The device, when temporarily attached to the discharge of a gas conduit, simulates a load on the system, producing an elevated pressure that can be used to estimate gas flow in the conduit. The device is then removed and the engine speed is set at the predetermined RPM during the unloading process. Accordingly, operators of dry bulk conveying or unloading systems can dependably and repeatedly set the proper blower speed for any variable speed blower to produce optimum conveying velocity for any dry-bulk product. The present invention works effectively regardless of variations in altitude, atmospheric conditions, blower frame size, power take-off ratio, and/or operator experience.
The present invention is advantageous because it has no moving parts and requires no calibration, maintenance, additional instrumentation or power, but nevertheless produces a repeatable indication (pressure) of the gas under field conditions. In spite of the simplicity of the present invention, it can still be used with variable-rate gas moving equipment, for example with blowers, fans, compressors or gas-flow throttling mechanisms, such as valves, dampers, and the like, to repeatedly and dependably establish accurate discharge values for that system.